How to keep two separate UV maps when merging two objects into one?
On each object, prior to merging, give it an additional UV map with the name of the other object's UV map.  After merging the objects, it will merge these individual UV maps.  Although note that when you create an additional UV map, it will have the same UV values as the selected UV map, so you might want to do whatever you want to these-- there's no such thing as being unmapped in some map, that just means UV 0,0.